This is a randomized, Phase III, multi-center, controlled study comparing the efficacy and safety of a single course of IDEC-Y2B8 with a course of Rituxan in patients with advanced, relapsed or refractory non-Hodgkin's B-cell lymphoma. A course of IDEC-Y2B8 includes an imaging dose of IDEC-In2B8 followed one week later by a single dose of IDEC-Y2B8; both IDEC-Y2B8 and IDEC-In2B8 are preceded by an infusion of 250 mg/m2 Rituxan. A course of Rituxan includes four infusions of 375 mg/m2 (once weekly x 4) of Rituxan. Patients with low-grade or follicular NHL who have relapsed disease or have failed primary conventional chemotherapy and now require treatment will be eligible for this study. Patients meeting the study inclusion and exclusion criteria may be enrolled into the study.